No.
1 - Winter 1984-85

The
Return of Halley's Comet

On February 9,
1986, the most famous comet of all time will make the closest approach to the
Sun in its current 76-year trip around our star. For a few months, before and
after that date, the comet will be visible from our vantage point on the Earth
— but, alas, not as well as it was in 1910. We begin the inaugural issue
of The Universe in the Classroom with an in-depth look at Comet
Halley (pronounced to rhyme with "Sally'') and its upcoming "not-so-close
encounter'' with the Earth.

Comets are literally
cosmic icebergs — chunks of ice with dust and rocky particles embedded in them.
This "dirty ice'' includes not just frozen water, but many other frozen substances
as well. A typical comet iceberg is only a few kilometers wide, very small compared
to most of the objects astronomers study.

According to our
modern theories, comets are frozen debris, left over from the formation of the
solar system. We believe there is a vast "cloud'' of icy pieces in the
outer regions of the solar system, far beyond the orbit of Pluto. While most
of these will never leave their distant orbits, a few are disturbed in their
motion and come into the inner solar system, revealing themselves to us.

These comets then
move around the solar system in huge elongated orbits, which means they spend
most of their time far away from the Sun. Out there a comet remains frozen and
is invisible to even our best telescopes. But as a comet's path brings it closer
and closer to the Sun, the heat begins to evaporate the ices and release some
of the dust particles trapped within them.

The freed gas and
dust particles form a cloud or "halo'' around the iceberg which astronomers
call a coma. A comet's coma can get as large as 100,000 km across as the comet
nears the Sun, and the glowing gases of the coma produce the "fuzzy patch''
appearance we associate with comets.

Not all the material
liberated from the comet iceberg remains in the coma. A flow of high speed particles
from the Sun (which astronomers call the "solar wind'') pushes on the gas
particles in the coma and sweeps them into a long "tail'' pointing away
from the Sun. In addition, the dust particles in the outer coma are pushed into
a gently curving dust tail by the pressure of the Sun's radiation.

Comet tails can
stretch out for more than 10 million kilometers. Yet they contain so few particles
in total that the material in a com- et tail could easily fit into an average
suitcase and leave enough room for a change of clothes.

The comets we see
follow a variety of paths across the expanse of our solar system. Some make a
single pass into the region of the Sun and then retum to the deep freeze of the
outer solar system, never to be seen again. Others may have long elliptical paths
that will bring them back to us, but only after many thousands or millions of
years have passed.

Some comets, on
the other hand, have had their orbits altered by the gravitational pull of the
giant outer planets — Jupiter, Saturn, Uranus, and Neptune. These "short-period
comets'' — of which about 100 are known — mostly stay within the region of
the planets and can thus grace our skies much more regularly.

Comet Halley is
a fine example of a short-period comet, taking about 76 years to make one orbit
around the Sun. At its closest point (perihelion), the comet is only 88 million
kilometers from the Sun (about 60% of the Earth's distance from our star.) At
its most distant point (aphelion), Halley is 5.2 billion kilometers from the
Sun and thus beyond the orbit of Neptune.

The
coma and tail of a comet, indicated on a photograph of Halley's Comet
taken on May 8, 1910. (Mt. Wilson Observatory photograph.)

The British astronomer
Edmond Halley (1656-1742) was using Isaac Newton's ideas of gravitation to analyze
the motion of bodies in the solar system. He noticed that the records for the
bright comets of 1531, 1607, and 1682 showed that all three comets had very similar
orbits. He drew the bold conclusion that all three were really the same comet,
trapped by the gravitational pull of the outer planets, and predicted that the
comet would retum in 1758-59. The comet was found again on Christmas night 1758
and was then named in the late astronomer's honor.

On October 16,
1982, a team of astronomers led by David Jewett and G. Edward Danielson recovered
Comet Halley using the 200-inch telescope on Palomar Mountain in Southern California.
Since the comet at that time was about fifty million times fainter than the
faintest objects our eyes can see, they had to use not only the largest American
telescope but also special electronic equipment which had been developed for
the upcoming Space Telescope.

When they found
the comet, it was about 1.6 billion kilometers from the Sun, still beyond the
orbit of Saturn. This recovery beats the previous record for finding Comet Halley
by over two years! The comet was almost exactly where astronomers predicted
it would be and right on time for its closest pass to the Sun on February 9,
1985.

Unfortunately, this
will be one of the less favorable appearances of the comet and significantly less
impressive than the one in 1910. How dramatic a comet appears to us on Earth depends
on several factors: how bright it really is, how close it is to the Earth when
it crosses our orbit, on which side of the Sun it makes its closest approach to
our star, and how high above the horizon it is when we see it. The news on many
of these fronts is not very promising.

On its way in toward
the Sun, Comet Halley will pass closest to the Earth on November 27, 1985, at
a distance of 93 million kilometers. On its outward journey, the comet will
come within 48 million kilometers of us on April 11,1986. In 1910, however,
the closest approach was within 24 million kilometers and we passed through
one part of the comet's tail. Therefore, the comet will look a lot fainter this
time around and some astronomers predict that in the bright skies of most cities
it may well be invisible to the naked eye.

To make things
a bit more complicated, the comet's orbit is tilted when compared to the orbits
of the planets. This means that the comet will be better seen in the Northern
Hemisphere as it approaches the Sun and in the Southern Hemisphere as it is
leaving. Since it will be closer to the Earth in the spring of 1986, Southern
Hemisphere observers get a better show this time around. But the best views
of the comet should come from spacecraft and orbiting observatories (see below).

The best way for
the average person to observe the comet will be to use a good pair of binoculars
in a location that is away from city lights and has a good view of the horizon
(since the comet will be low in the sky). 7x50 or 7x35 binoculars should provide
the best views.

For much of January
1986, the comet should be faintly visible low in the western sky for a short while
after sunset. Then it will disappear behind the Sun and re-emerge from the Sun's
glow in the morning sky after the first week of March. Since the comet will be
favoring the Southern Hemisphere at this time, the farther south you are, the
better your view will be.

During the second
week in April the comet will be very low in the predawn sky and impossible for
many of us in the U.S. to see. It will become visible later in April in the
evening sky and will be getting fainter and fainter as the month goes on.

We will have much
more detailed observing guides and suggestions in this newsletter as the comet
comes closer.

Astronomers believe
that comets formed at the same time as our solar system, almost 5 billion years
ago. Here on Earth (and on many of the other planets), geological processes, volcanoes,
and weather long ago erased all traces of this remote epoch that gave us birth.

Comets, on the
other hand, spend most of their time in the "deep-freeze'' of the outer
solar system. The material in the comet "iceberg'' is thus well preserved
and could tell us a great deal about what things were like in that ancient time.

Astronomers can
also use comets as probes of the present conditions in the solar system. By
watching a comet's coma and tail develop and change, we can learn about the
flow of particles and energy from the Sun, the details of the magnetic fields
and particles between the planets, and the detailed characteristics of the comets
themselves.

To coordinate observations
of the comet around the world, astronomers have formed The International Halley
Watch, con sisting of professional and amateur astronomers in many countries.
All observations will be sent to IHW to establish a single archive that will be
the richest record of a cometary encounter ever assembled.

Not only will Halley
be observed with all sorts of telescopes on Earth, the advent of the space age
makes it possible for the first time to send space probes to make close-up measurements
of the comet's properties. A small armada of spacecraft will monitor Halley,
including five that will fly by the comet in March of 1986. Japan and the Soviet
Union are each sending two probes to Halley, while the European Space Agency
(the European equivalent of NASA) is sending one. The U.S. decided not to send
a fly-by craft to Halley, but we are redirecting an older probe to fly by another
comet (Giacobini-Zinner) in September 1985. In addition we will be making observations
of Comet Halley with a special package of instruments called Astro 1, to be
carried aloft by the Space Shuttle in early March of 1986.

Before we understood
the nature and orbits of comets, people worried about the effects a comet might
have on us. Today we know enough about these objects to provide a reassuring
"environmental impact statement.'' Unless a comet physically collides with
the Earth (which Halley will be far from doing), these small chunks of dirty
ice pose no danger to us. Their effects are on the mind, stimulating our curiosity
and kindling our imaginations.